CATV digital transmission with BandPass sampling

ABSTRACT

Methods and systems capable of improving the transmission of data along an upstream path of a Hybrid Fiber-Coaxial Cable Network, from a transmitter in a node to a receiver in a Cable Modem Termination System.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. 119(e) of priorityof U.S. Provisional Application No. 61/862,670, as filed on Aug. 6,2013.

BACKGROUND

The present disclosure relates to systems and methods that provide videoand data over a cable transmission network.

Referring to FIG. 1, cable TV (CATV) systems were initially deployed asvideo delivery systems. In its most basic form the system received videosignals at the cable head end, processed these for transmission andbroadcast them to homes via a tree and branch coaxial cable network. Inorder to deliver multiple TV channels concurrently, early CATV systemsassigned 6 MHz blocks of frequency to each channel and FrequencyDivision Multiplexed (FDM) the channels onto the coaxial cable RFsignals. Amplifiers were inserted along the path as required to boostthe signal and splitters and taps were deployed to enable the signals toreach the individual homes. Thus all homes received the same broadcastsignals.

As the reach of the systems increased, the signal distortion andoperational cost associated with long chains of amplifiers becameproblematic and segments of the coaxial cable were replaced with fiberoptic cables to create a Hybrid Fiber Coax (HFC) network to deliver theRF broadcast content to the coaxial neighborhood transmission network.Optical nodes in the network acted as optical to electrical convertersto provide the fiber-to-coax interfaces.

As the cable network evolved, broadcast digital video signals were addedto the multiplexed channels. The existing 6 MHz spacing for channels wasretained but with the evolving technology, each 6 MHz block could nowcontain multiple programs. Up to this point, each home received the sameset of signals broadcast from the head end so that the amount ofspectrum required was purely a function of the total channel count inthe program line-up.

The next major phase in CATV evolution was the addition of high speeddata service, which is an IP packet-based service, but appears on theHFC network as another 6 MHz channel block (or given data servicegrowth, more likely as multiple 6 MHz blocks). These blocks use FDM toshare the spectrum along with video services. Unlike broadcast video,each IP stream is unique. Thus the amount of spectrum required for dataservices is a function of the number of data users and the amount ofcontent they are downloading. With the rise of the Internet video, thisspectrum is growing at 50% compound annual growth rate and puttingsignificant pressure on the available bandwidth. Unlike broadcast video,data services require a two-way connection. Thus, the cable plant had toprovide a functional return path. Pressure on the available bandwidthhas been further increased with the advent of narrowcast video servicessuch as video-on-demand (VOD), which changes the broadcast video modelas users can select an individual program to watch and use VCR-likecontrols to start, stop, and fast-forward. In this case, as with dataservice, each user requires an individual program stream.

Thus, the HFC network is currently delivering a mix of broadcast video,narrowcast video, and high speed data services. Additional bandwidth isneeded both for new high definition broadcast channels and for thenarrowcast video and data services. The original HFC network has beensuccessfully updated to deliver new services, but the pressure of HD andnarrowcast requires further change. The HFC network is naturally splitinto the serving areas served from the individual fiber nodes. Thebroadcast content needs to be delivered to all fiber nodes, but thenarrowcast services need only be delivered to the fiber node serving thespecific user. Thus, there is a need to deliver different service setsto each fiber node and also to reduce the number of subscribers servedfrom each node (i.e. to subdivide existing serving areas and thusincrease the amount of narrowcast bandwidth available per user).

FIG. 1 is a generalized representation of part of the cable TVinfrastructure, which includes the cable head end; the Hybrid Fiber Coax(HFC) transmission network, and the home. The CATV head end receivesincoming data and video signals from various sources (e.g., fiber opticlinks, CDN's, DBS satellites, local stations, etc.). The video signalsare processed (reformatting, encryption, advertising insertion etc.) andpackaged to create the program line up for local distribution. This setof video programs is combined with data services and other systemmanagement signals and prepared for transmission over the HFC to thehome. All information (video, data, and management) is delivered fromthe head end over the HFC network to the home as RF signals. In thecurrent practice, systems in the head end process the signals, modulatethem to create independent RF signals, combine these into a singlebroadband multiplex, and transmit this multiplex to the home. Thesignals (different video channels and one or more data and managementchannels) are transmitted concurrently over the plant at different FDMfrequencies. In the home, a cable receiver decodes the incoming signaland routes it to TV sets or computers as required.

Cable receivers, including those integrated into set-top boxes and othersuch devices, typically receive this information from the head end viacoaxial transmission cables. The RF signal that is delivered cansimultaneously provide a wide variety of content, e.g. high speed dataservice and up to several hundred television channels, together withancillary data such as programming guide information, ticker feeds,score guides, etc. Through the cable receiver's output connection to thehome network, the content is delivered to television sets, computers,and other devices. The head end will typically deliver CATV content tomany thousands of individual households, each equipped with a compatiblereceiver.

Cable receivers are broadly available in many different hardwareconfigurations. For example, an external cable receiver is oftenconfigured as a small box having one port connectable to a wall outletdelivering an RF signal, and one or more other ports connectable toappliances such as computers, televisions, and wireless routers or othernetwork connections (e.g., 10/100/1,000 Mbps Ethernet). Other cablereceivers are configured as circuit cards that may be insertedinternally in a computer to similarly receive the signals from an RFwall outlet and deliver those signals to a computer, a television, or anetwork, etc. Still other cable receivers may be integrated into set-topboxes, such as the Motorola DCX3400 HD/DVR, M-Card Set-Top, whichreceives an input signal via an RF cable, decodes the RF signal toseparate it into distinct channels or frequency bands providingindividual content, and provides such content to a television or otheraudio or audiovisual device in a manner that permits users to eachselect among available content using the set top box.

As previously mentioned, the CATV transmission architecture has beenmodified to permit data to flow in both directions, i.e. data may flownot only from the head end to the viewer, but also from the viewer tothe head end. To achieve this functionality, cable operators dedicateone spectrum of frequencies to deliver forward path signals from thehead end to the viewer, and another (typically much smaller) spectrum offrequencies to deliver return path signals from the viewer to the headend. The components in the cable network have been modified so that theyare capable of separating the forward path signals from the return pathsignals, and separately amplifying the signals from each respectivedirection in their associated frequency range.

The Hybrid/Fiber Coax (HFC) cable network architecture broadly depictedin FIG. 1 includes a head end system 10 having multiple devices fordelivery of video and data services including EdgeQAMS (EQAMs) forvideo, cable modem termination systems (CMTS) for data, and otherprocessing devices for control and management. These systems areconnected to multiple fiber optic cables 12 that go to variousneighborhood locations that each serve a smaller community. A fiberoptic neighborhood or multi-neighborhood node 14 is located between eachfiber optic cable 12 and a corresponding trunk cable 16, which in turnis interconnected to the homes 20 through drop cables 18 and feedercables (not shown). Because the trunk cable 16, as well as the branchnetworks and feeder cables 18, each propagate RF signals using coaxialcable, the nodes 14 convert the optical signals to electrical signalsthat can be transmitted through a coaxial medium, i.e. copper wire.Similarly, when electrical signals from the home reach the node 14 overthe coaxial medium, those signals are converted to optical signals andtransmitted across the fiber optic cables 12 back to the systems at thehead end 10. The trunk cables 16 and/or feeder cables 18 may includeamplifiers 17. Connected to each trunk cable 16 is a branch network thatconnects to feeder cables (or taps) that each enter individual homes toconnect to a respective cable receiver.

Hybrid fiber/coax networks generally have a bandwidth of approximately750 MHz or more. Each television channel or other distinct content itemtransmitted along the forward path from the head end to a user may beassigned a separate frequency band, which as noted earlier has a typicalspectral width of 6 MHz. Similarly, distinct content delivered along thereturn path from a user to the head end may similarly be assigned aseparate frequency band, such as one having a spectral width of 6.4 MHz.In North America, the hybrid fiber/coax networks assign the frequencyspectrum between 5 MHz and 42 MHz to propagate signals along the returnpath, and assign the frequency spectrum between 50 MHz and 750 MHz ormore to propagate signals along the forward path.

Referring to FIG. 2, a cable modem termination system (CMTS) 30 may beinstalled at the head end, which instructs each of the cable modems whento transmit return path signals, such as Internet protocol (IP) basedsignals, and which frequency bands to use for return path transmissions.The CMTS 30 demodulates the return path signals, translates them backinto (IP) packets, and redirects them to a central switch 32. Thecentral switch 32 redirects the IP packets to an IP router 34 fortransmission across the Internet 36, and to the CMTS which modulatesforward path signals for transmission across the hybrid fiber coaxcables to the user's cable modem. The central switch 32 also sendsinformation to, and receives information from, information servers 38such as video servers. The central switch 32 also sends information to,and receives information from, a telephone switch 40 which isinterconnected to the telephone network 42. In general, cable modems aredesigned to only receive from, and send signals to, the CMTS 30, and maynot communicate directly with other cable modems networked through thehead end.

FIG. 3 shows an exemplary architecture for delivering CATV contentbetween a head end 10 to a node 14. The head end 10 may in someinstances include a plurality of direct modulation EdgeQAM units 50which each receive digitally encoded video signals, audio signals,and/or IP signals, and each directly outputs a spectrum ofamplitude-modulated analog signal at a defined frequency or set offrequencies to an RF combining network 52, which in turn combines thereceived signals. An optical transmitter 54 then sends the entirespectrum of the multiplexed signals as an analog transmission through anoptical fiber network 56 along a forward path to the node 14. The fiberoptic network, as will be explained in more detail later, is alsocapable of conveying optical signals from the node 14 to the head end 10via an optical path between a transmitter 58 in the node 14 and areceiver 60 in the head end. In the specification, the drawings, and theclaims, the terms “forward path” and “downstream” may be interchangeablyused to refer to a path from a head end to a node, a node to anend-user, or a head end to an end user. Conversely, the terms “returnpath”, “reverse path” and “upstream” may be interchangeably used torefer to a path from an end user to a node, a node to a head end, or anend user to a head end. Also, it should be understood that, unlessstated otherwise, the term “head end” will also encompass a “hub,” whichis a smaller signal generation unit downstream from a head end, oftenused for community access channel insertion and other purposes, thatgenerally mimics the functionality of a head end, but may typically notinclude equipment such as satellite dishes and telephone units. Hubs arecommonly known to those skilled in the art of the present disclosure. Itshould be understood that although FIG. 3 illustrates a head end 10 thatutilizes direct modulation EdgeQAMs, other architectures may employother modulators, such as an analog EdgeQAM modulator or a ConvergedCable Access Platform (CCAP) modulation system.

Directly-modulated EdgeQAM units have become increasingly sophisticated,offering successively higher densities, which in turn means that eachEdgeQAM unit can process more channels of CATV data. For example, modernEdgeQAM modulation products can now simultaneously generate 32 or morechannels on a single output port. With more channels being modulated peroutput port, the amount of combining required by the RF combiningnetwork 52 is reduced, with a corresponding simplification in thecircuitry at the head end. The term ‘QAM’ is often used tointerchangeably represent either: (1) a single channel typically 6 MHzwide that is Quadrature Amplitude Modulated (thus a “32 QAM system” isshorthand for a system with 32 Quadrature Amplitude Modulated channels;or (2) the depth of modulation used by the Quadrature AmplitudeModulation on a particular channel, e.g. 256 QAM means the signal ismodulated to carry 8 bits per symbol while 4096 QAM means the signal ismodulated to carry 12 bits per symbol. A higher QAM channel count or ahigher QAM modulation means that a higher number of content “channels”can be delivered over a transmission network at a given standard ofquality for audio, video, data, etc. QAM channels are constructed to be6 MHz in bandwidth in North America, to be compatible with legacy analogTV channels and other existing CATV signals. However, more than onevideo program or cable modem system data stream may be digitally encodedwithin a single QAM channel. The term channel is unfortunately oftenused interchangeably, even though a QAM channel and a video program arenot often the same entity—multiple video programs can be and usually areencoded within a single 6 MHz QAM channel. In this case, the modernEdgeQAM modulation products generate multiple instances of the 6 MHzbandwidth QAM channels. This simplifies the head end structure sincesome subset of the RF combining is now performed within the EdgeQAMunits rather than in the external RF combining network. Packagingmultiple QAM generators within a single package also offers someeconomic value.

As noted previously, modern CATV delivery systems over an HFC networkprovides content that requires communication along both a forward pathand a return path, and over time, the quantity and quality of datatransmission along each of these paths has increased drastically, whichcan be seen for example in the evolution of the DOCSIS standard from itsoriginal 1.0 release to the impending 3.1 release.

DOCSIS (Data Over Cable Service Interface Specifications) was developedby a consortium of companies, including Cable Labs, ARRIS, Cisco,Motorola, Netgear, and Texas Instruments, among others. The firstspecification, version 1.x, was initially released in March 1997 andcalled for a downstream throughput of approximately 43 Mbps and anupstream throughput of approximately 10 Mbps along a minimum of onechannel. DOCSIS 2.0, released in late 2001 increased the maximumupstream throughput to approximately 31 Mbps, again for a minimum of onechannel. DOCSIS 3.0, released in 2006 required that hardware be able tosupport the DOCSIS 2.0 throughput standards of 43 Mbps and 31 Mbps,respectively, along minimum of four channels in each direction. TheDOCSIS 3.1 platform is aiming to support capacities of at least 10 Gbpsdownstream and 1 Gbps upstream using 4096 QAM. The new specificationaims to replace the 6 MHz downstream and 6.4 MHz upstream wide channelspacing with smaller 25 kHz to 50 kHz orthogonal frequency divisionmultiplexing (OFDM) subcarriers, which can be bonded inside a blockspectrum that could end up being about 192 MHz wide for downstream and96 MHz for upstream.

Providing increasing throughput along the upstream path has beenparticularly problematical the presence of upstream impairmentsincluding ingress. Ingress is radio frequency (RF) energy that hasvarying bandwidth and RF levels and can enter the CATV upstream plantvia cable network defects. CATV network defects may include loose orcorroded connectors, unterminated ports, and damaged cables, forexample. Thus, to continue to meet the evolving needs of delivering CATVcontent, improved techniques for transmission along the upstream path,and particularly in the presence of upstream impairments. are desirable.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an exemplary Hybrid/Fiber Coax CATV network including ahead end that delivers CATV content to a plurality of homes.

FIG. 2 shows an exemplary architecture of a head end, such as the onesshown in FIG. 1.

FIG. 3 shows an exemplary EdgeQAM architecture for a head end tocommunicate with a node along a forward path to deliver CATV contentover a network.

FIG. 4 shows an example of an upstream transmission system according toone aspect of the present disclosure.

FIG. 5 shows a band-limited signal capable of being sampled at a rate ofless than twice its upper cutoff frequency while still preserving allits information.

FIG. 6 shows a brassboard DRR output spectrum using existing 2×85Digital Return Transmitter (DRT) and Digital Return Receiver (DRR)systems.

FIG. 7 shows an exemplary architecture including the system shown inFIG. 1.

FIGS. 8 and 9 each show respective examples of DOCSIS 3.0 capacityaggregation in a node.

DETAILED DESCRIPTION

FIG. 4 generally illustrates an improved system for transmitting dataalong a return path from a Digital Return Transmitter (DRT) 107, in anode for example, to a Digital Return Receiver (DRR) 109 in a head end,for example. Specifically, FIG. 4 shows a DRT 107 receiving two signals102 and 104 for transmission to the DRR 109. Future DOCSIS channels mayoccupy a maximum bandwidth of 96 MHz, approximately 15 times larger thanthe largest upstream DOCSIS 3.0 channel, where each of the signals 102and 104 can generate a digital signal rate anywhere in the range of 800Mbps to 2.25 Gbps depending on the signal band, sampling rate and thenumber of bits used to sample each signal. Even when digitizing only twoof these signals, and transmitting them both together using timedivision multiplexing of the digital signals, the aggregated digitalsignal rate may exceed the capabilities of existing digital and opticaltransport platforms.

The system of FIG. 4, however, permits the signals 102 and 104 to besimultaneously transmitted using existing architectures by takingadvantage of the principles of bandpass sampling. Ordinarily, theNyquist sampling theorem dictates that in order to completely preservethe information in a transmitted signal occupying a limited portion ofthe frequency spectrum, the signal must be sampled at a rate equal totwice the upper frequency limit of the signal. For example, if themaximum frequency of the signal is 100 MHz, the signal must be sampledat 200 million samples per second in order to preserve all theinformation in the signal. At even modest symbol bit rates of say six toeight bits per sample, the required throughput to sample at the fullNyquist rate can become significant.

FIG. 5, however, illustrates a circumstance in which all information ina transmitted signal may be preserves without sampling at the rate oftwice the upper cutoff frequency of the signal. Specifically, a signal140 a, 140 b may be bandlimited to a 20 MHz segment of the frequencyspectrum from plus or minus 90 MHz to plus or minus 110 MHz. It will beunderstood by those skilled in the art that the signal need only besampled on the positive sideband, as the negative sideband is simply amirror image of the positive sideband. FIG. 5 also shows spectralaliases 142 of the signal 140 a that extend from the baseband to thesignal component 140 a. Stated differently, a theoretical repeatingsignal occupying a single frequency on the spectrum at 100 MHz, thusrepeating 100 million times per second, has a spectral alias occurringat every 1 Hz interval. By extension, the signal 140 a has spectralaliases 142 that begin at baseband and repeat at every integral multipleof the width of the signal 140 a. If signal 140 a is located on thespectrum at an integral multiple of its width, the spectral aliases donot overlap, and thus the signal 140 a may be fully sampled by samplingits baseband spectral alias only, i.e. by sampling at a rate of twicethe spectral width signal 140 a rather than twice its upper cutofffrequency. If the signal 140 a is located on the spectrum at a positionthat is not an integral multiple of its width, the spectral aliases willoverlap, but the signal 140 a may be fully sampled by sampling at a ratethat is greater than twice its spectral width, but still much less thantwice its upper cutoff frequency.

As noted previously, existing hybrid fiber coax architectures includecomponents that would ordinarily be considered as lacking the capabilityof processing two or more signals 102 and 104 sequentially positioned onthe frequency position, if those signals conformed to anticipated futureDOCSIS standards. The present inventors realized, however, that bytaking advantage of bandpass sampling principles, existing architecturescould be modified without the need to upgrade much of the equipment inexisting architectures.

Referring again to FIG. 4, the disclosed system may preferably sampleand transmit plural wideband orthogonal frequency division multipleaccess (OFDM) channels 102 and 104, each with up to 96 MHz BW. It shouldbe understood, however, that other embodiments may simultaneouslytransmit more than two OFDM channels, such as four channels for example,guardband 106 between channels may be included to prevent interferencebetween the channels. In this example, a fully loaded DOCSIS return mayrequire an upper frequency of >200 MH, e.g. a lower OFDM channel 102from 10-106 MHz and a upper OFDM channel 104 from 126-222 MHz. Todigitize the continuous band from 0 to 222 MHz, the sampling rate wouldneed to be >444 Msps (likely ˜510 Msps), well in excess of devices usedin current 2×85 digital return.

A DRT 107 may first filter the signals 102 and 104 with a diplex filter108 that separates the signals 102, 104 into a first transmission path109 for the signal 104 and a second transmission path 111 for the signal102. The diplex filter preferably splits the signals 102 and 104 at asplit frequency of approximately 116 MHz. The term “approximately inthis context means anywhere within a range of 106 MHz to 126 MHz, on theassumption that each of the signals 102 and 104 are about 96 MHz inwidth and the guardband is about 20 MHz in width. Alternativeembodiments may use a split frequency outside of this range, however,depending on the width of the signals 102 and 104. The signal 102 ispreferably filtered by a low pass filter 110 while the signal 104 ispreferably filtered by a bandpass filter 112. Preferably, each signal102 and 104 may be amplified after being respectively filtered. Thebandpass filter 112 is preferably configured to pass a range offrequencies that closely matches the frequency bounds of the signal 104,and to filter out all other frequencies. After each signal 102, 104 isfiltered by a respective one of the filters 110 and 112, the signals 102and 104 are converted to a digital signal by a dual A/D converter 114.The dual A/D converter 114 can be operated with independent analoginputs and/or can be used for diversity reception of signals, operatingidentically on the same carrier but from separate antennae. The outputfrom the dual A/D converter 114 is input to a multiplexer 116, a devicethat selects one of several analog or digital input signals and forwardsthe selected input into a single line.

As can be seen in FIG. 4, between signal 102 and signal 104 is aguardband 106, which accounts for the fact that the diplex filter 108has a significant cross-over region, in which frequencies above thelowpass portion of the diplex filter get passed, and frequencies belowthe highpass portion of the diplex filter get passed. The guardband,though necessary to protect the integrity of the respective signals 102and 104, contains no data, hence the present inventors recognized thatthere is no reason for sampling frequencies within the guardband. Thus,the dual A/D converter is preferably configured to only sample therespective bands occupied by the signals 102 and 104, and withoutsampling the frequencies that occupy the guardband.

As can easily be recognized, by using bandpass filter 112 in conjunctionwith the disclosed dual A/D converter 114, a signal with bandwidth of 96MHz can be sampled at >192 Msps (likely ˜232 Msps), close to the current2×85 sampling rate of 202 Msps. Thus, the use of bandpass samplingtransforms the 2×85 Digital Return into a 1×200+ Digital Return. The useof digitized blocks to leverage the DSP concept of bandpass sampling mayachieve strict Nyquist compliance, aggregating the signals on thereceive end.

In some embodiments, a single hardware design can support both 2×85 MHzand 1×200+MHz. Thus, one hardware design can support both 2×85 MHz and1×200+MHz, avoiding a total re-design of the 2×85 MHz legacy designs,which would be more complex and costly. In the DOCSIS digital returnimplementation shown in FIG. 4, an increase in current digitalprocessing rates, e.g., from 202 to 232 Msps, supports the two (2) 96MHz bandwidth channels 102 and 104. For example, one of the currenttime-division multiplexing (TDM) channels may carry the lower 96 MHzDOCSIS 3.1 channel 102 via conventional lowpass sampling, such as vialow pass filter 110. The second of the TDM channels may carry the upper96 MHz DOCSIS 3.1 channel 104 via bandpass sampling using bandpassfilter 112.

As shown in FIG. 4, by bandpass sampling the advanced signaling on aDOCSIS channel, ingress impairments may be filtered out from the opticaltransmission path, since the filter characteristics are closely matchedto the advanced signaling channel, e.g. the 96 MHz orthogonal frequencydivision multiple access (OFDMA signal) proposed by the DOCSIS 3.1standard.

Preferably, the bandpass filter characteristics closely match theadvanced signaling channel (or channels), thereby filtering out ingressimpairments from the optical transmission path. Thus, the disclosedtechniques may address modern cable upstream issues in a more economicalmanner than existing architectures. For example, in a Data Over CableService Interface Specifications (DOCSIS) implementation, the disclosedtechniques may enable DOCSIS over extended splits by using therelationship between the state of the art in upstream digital returnproducts (2×TDM @ 85 MHz split) and newly defined upstream approaches,e.g., using 2×96 MHz bands of OFDMA.

It should be understood that other center frequencies, bandwidths, andaggregation schemes can be chosen to provide desired performance. Forexample, if a signal band is not completely full, all the dynamic range(sampling rate) can be given to the occupied bandwidth to achieve abetter SNR ratio. Similar techniques can be used to protect dynamicrange and/or avoid overdrive due to regions of significant interferenceand ingress that are unoccupied with desired signals, such as the lowend of the upstream band, or perhaps the FM band when upstream signalcapacity is extended even further beyond DOCSIS 3.1.

Also, the disclosed techniques apply to any future extension of upstreambandwidth capacity—the tradeoff between full band digitization,processing, and transport versus aggregated spectrum increments. Whilethe summation of summing spectrum segments to a total bandwidth islinear, the implementation costs of summing spectrum components thatmatch a full single bandwidth approach may not be linear.

The multiplexer 116 may combine several input signals into one outputsignal, which carries several communication channels. The multiplexer116 may increase the amount of data that can be sent over the channelwithin a certain amount of time and bandwidth. The multiplexer 116 makesit possible for the bandpass filter and low pass filter to share theDual A/D 114. The MUX and Framing FPGA 116 may provide the output to aserializer and Electrical-to-Optical (E20) converter 118.

A Digital Return Receiver (DRR) 109 may included anOptical-to-Electrical (O2E) converter and deserializer component 120that provides an output to a deframe and demultiplexer FPGA 122. Thedemultiplexer 122 may take a single input signal and select one of manydata output-lines connected to a single input. The demultiplexer 122 maytake a single input signal that carries many channels and separate thoseover multiple output signals for delivery to a Dual A/D converter 124,where a demultiplexed first output signal 125 can be delivered to abandpass filter 126 and a second output signal 127 is provided to a lowpass filter 128. The signals 125 and 127 may be combined by a diplexfilter/combiner 130 to re-create the original signals 102 and 104.

The center frequencies and bandwidths described herein are non-limitingexamples of trading between full band digitization, processing andtransport vs. the disclosed aggregated spectrum increments. For example,other center frequencies and bandwidths and aggregation schemes can bechosen to achieve this result to provide other performance advantages.In particular, for example, if the band is not completely full, all ofthe dynamic range can be given to the occupied BW to optimizeperformance (better SNR). This same mechanism also can be used toprotect the dynamic range and/or avoid overdrive due to regions ofsignificant interference and ingress that are unoccupied with desiredsignals, such as the low end of the upstream band or perhaps the FM bandwhen the upstream is extended such as is anticipated by this disclosure.

Further, the disclosed concepts of separately digitizing single channelsapplies to future upstream bandwidth and recognizes the tradeoff betweenfull band digitization, processing, and transport vs. aggregatedspectrum increments. While the algebra of summing spectrum segments to atotal BW required is linear, the implementation costs of summingspectrum components that match a full single-BW approach may not belinear, in particular when processing is pushing the technologyenvelope, which has been inherently the situation recently in thecompetitive environment of broadband evolution. Existing solutions donot make use of bandpass sampling in the area of digital return; inparticular, the disclosed techniques for using bandpass sampling, adigital signal processing technique, to enable flexible selection ofcritical system parameters including bandwidth, and center frequencylocation, have not been contemplated. Dynamic selection of thesecritical system parameters enables both efficient transmission of CATVupstream paying services and avoidance of upstream impairments includingingress and laser clipping.

FIG. 6 depicts a brassboard DRR output spectrum using existing 2×85 DRTand DRR boards. In this example, 85 MHz analog lowpass filters in DRTand DRR are bypassed in one of the TDM channels. Analog amps currentlyused have decreasing gain from 85 to 200 MHz and therefore the passbandslope and ripple may be further optimized. Sampling and processing ratehas not been bumped, in this example, still running at a rate of 202Msps. Due to keeping rate at 202 Msps, in this embodiment, two ˜80 MHzBW channels are used rather than two (2) 96 MHz channels. In thisembodiment, the lower channel is 5-85 MHz and the upper channel is113-197 MHz. An 80 MHz noise block to simulate lower wideband channeland fourteen 256 QAMs is used to simulate upper wideband channel.Signals are being passed with respectable SNR and distortions but MER athigher frequencies is being limited by phase jitter.

In embodiments, phase jitter and phase noise requirements are determinedfor the clocks to support 42+dB MER at the upper end of the band, whichmay dictate hardware design choices to meet the phase jitterrequirements. Further, analog amps and filters may be designed tosupport upper wideband channel and optimize passband response. Inembodiments where the sampling rates are bumped from 202 to 232 MHz, thephase jitter and phase noise requirements may be determined. The DRT andDRR boards may be configured to deliver >42 dB MER at high end of band.

FIG. 7 depicts an exemplary architecture 150 by which data may betransmitted along a return path between a plurality of end user cablemodems 152 and a CMTS 156 in a head end 157, through a node 154. In thisarchitecture, a DRT 158 in the node receives respective signals fromeach of the modems 152 via an array of coaxial cables 160. The coaxialcables may be connected to modems 152 using two-way splitters used ascombiners 162 such that a respective two-way splitter 162 propagatessignals onto a coaxial cable 160 from a pair of modems 152. Thetransmitter 158 is preferably configured as shown in FIG. 4, andpropagates a digital signal onto a fiber optic transmission line 164 attwo wavelengths, and which is carried to a receiver 166 that is alsopreferably configured as shown in FIG. 4. After converting the incomingdigital signals to respective analog RF signals, the receiver 166 mayconvey them to the CMTS 156 using coaxial cables 168 and two-waysplitters 170.

It should be understood that, although FIG. 4 only shows an architecturefor providing data along an upstream path from the cable modems 152 tothe CMTS 156, such an architecture also includes equipment to providedata along the forward path from the CMTS 156 to the node 145 and on tothe cable modems 152.

FIG. 8 shows estimates in a system similar to that of FIG. 6, butwithout using the bandpass filter 112 of FIG. 4. In this example, datais delivered along an upstream path to the node by a coaxial cablecarrying 85 MHz blocks of data. In this example, the throughput orcapacity at the node was estimated to range from approximately 307 Mbpsto 983 Mbps depending on the number of channels and/or the depth ofmodulation, e.g. 64 QAM versus 256 QAM.

FIG. 9 shows estimates in a system similar to that of FIG. 6, butinstead using the bandpass filter 112 of FIG. 4, under several channelconditions presumed to be consistent with the upcoming DOCSIS 3.1standard, e.g. up to 1024 QAM while still using 85 MHz blocks. In thisexample, the throughput or capacity at the node was measured to rangefrom approximately 491 Mbps to 1,536 Mbps depending on the number ofchannels and/or the depth of modulation, e.g. 256 QAM versus 1024 QAM.As can be seen from a comparison between FIGS. 8 and 9, the use of abandpass filter and bandpass sampling as disclosed in the presentapplication approximately doubles the upstream throughput in a CATVtransmission system using lower complexity hardware.

In some embodiments, the disclosed techniques may be implemented via anincremental complexity addition to a current digital return product,e.g., an >85 MHz digital return product. Incremental complexity additionto existing products may enable upgrades, e.g., >85 MHz CATV upstreamupgrades, at an incremental cost to cable system operators, therebyavoiding the use of more expensive analog-to-digital converters (ADCs)associated with sampling equivalent frequency ranges. In an exampledigital return device, real estate in the node housing of an SG4 opticalhub would enable cable operators to easily aggregate capacities as highas 6 GB per node as needed.

The disclosed techniques provide flexibility to support dynamic centerfrequency locations. The ability to support dynamic center frequencylocations may allow cable operators the flexibility to focus opticallink transmission on paying services, rather than the traditionalapproach of transmitting both paying services plus ingress.

The terms and expressions that have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theclaimed subject matter is defined and limited only by the claims thatfollow.

The invention claimed is:
 1. A transmitter for transmitting signalsupstream in a bidirectional cable television (CATV) system, thetransmitter comprising: an input capable of receiving return pathsignals from a plurality of channels sharing a spectrum, wherein atleast one of the plurality of channels is a lower channel and theplurality of channels is an upper wideband orthogonal frequency divisionmultiple access (OFDM) channel; a plurality of filters for separatelyfiltering return path signals sharing a first spectrum; a processingdevice for increasing a throughput of the first spectrum by operatingtogether with the plurality of filters to sample and digitize signalsreceived from the plurality of channels for simultaneous transmission inthe first spectrum, wherein the plurality of filters are bandpassfilters for bandpass sampling return path signals from the wideband OFDMchannel by sampling the return path signals at a rate of at least twicethe spectral width signal; wherein a diplex filter is included thatsends a first filtered signal to at least one of the bandpass filters,wherein the diplex filter sends a second filtered signal to a lowpassfilter and where the second filtered signal is not sent to the bandpassfilters; wherein the processing device is a dual AID converter thatsamples and diaitizes the first filtered signal after it has passedthrough at least one of the bandpass filters, and samples and digitizesthe second filtered signal after it has passed through the lowpassfilter.
 2. The transmitter of claim 1 where a second portion of thefirst spectrum is not sampled.
 3. The transmitter of claim 1, where thediplex filter splits an input signal at a split frequency ofapproximately 116 Mhz.
 4. The transmitter of claim 1 capable oftransmitting a signal that complies with the DOCSIS 3.1 standard.